1. Field of the Invention
This invention relates to an optical receiver circuit, and more particularly, to an electrical circuit which includes an AC coupling circuit and a differential transimpedance amplifier for generating a digital data stream from a radiant energy signal.
2. Description of the Prior Art
A typical high-speed optical receiver design uses a photodetector to convert the optical signal to a current signal. The current signal is coupled to the input of a transimpedance amplifier which converts the current signal to a voltage signal. One example of a transimpedance amplifier is described in U.S. Pat. No. 4,498,001 which issued on Feb. 5, 1985 to L. S. Smoot. The transimpedance amplifier of '001 has an increased dynamic range. A peak detector, at the output of an inverting amplifier, turns on a field effect transistor when an AC component of an electrical signal becomes so large that the inverting amplifier would otherwise go into saturation. The field effect transistor circuit acts as an AC shunt impedance at the input of the inverting amplifier and diverts the excess AC current to ground. In addition, sense and sink current mirrors effectively divert an excessive DC component of the electrical signal away from the input of the inverting amplifier. The optical sensitivity and performance of the optical receiver remains unchanged while the dynamic range of the transimpedance amplifier is increased.
AC coupling of a current signal to the input of a transimpedance amplifier is required where a small signal may have a varying or large DC component that must be subtracted from the current signal before it can be accurately amplified. AC coupling of a current signal is also required when a differential transimpedance amplifier is used. Further, AC coupling must be used when a large voltage bias is applied across the photodetector and the current signal is to be connected to the input of a differential amplifier.
Referring to FIG. 1, an optical receiver 10 of the prior art is shown comprising a photodetector 12, an AC coupling input stage 14 and a differential transimpedance amplifier 16. The DC bias voltage across photodetector 12 is removed by identical capacitors 17 and 18. The low cutoff frequency is determined by the RC time constant of resistor 19 and capacitor 17 and by resistor 20 and capacitor 18. Large values of resistors 19 and 20 and capacitors 17 and 18 are desired in order that a sufficiently low cutoff frequency is attained. However, in monolithic integration or in an integrated circuit chip, the coupling capacitance, i.e. capacitors 17 and 18, cannot be made very large due to area constraints on the integrated circuit chip and also due to the large parasitic capacitances associated with capacitors formed on integrated circuit chips. Large parasitic capacitances lower the bandwidth and also decrease the sensitivity of the amplifier arrangement. Large values of biasing resistances, resistors 19 and 20 are desired. To maintain a large value of resistors 19 and 20 while maintaining sufficient voltage bias across photodetector 12 for large input currents, current sources may be used in place of resistances 19 and 20. The current sources maintain an average of the photodetector current. This ensures that the small-signal low cutoff frequency is sufficiently low regardless of input signal. However, for large signal input currents, a string of a few 1's or 0's causes the photodetector node voltages to approach power supply voltages, thus eliminating the input signal to the amplifier. Alternatively, if the bias voltage across detector 12 is reduced, the high-speed operation of optical receiver circuit 10 is compromised. As an example, consider a +2 dBm input signal causing a photodetector current of approximately 2 mA. For coupling capacitors 17 and 18 having a value of 10 pF, the bias voltage across photodetector 12 changes by 0.4 V/nanosecond. Coupling capacitors of only 10 pF in prior art circuits is a severe limitation particularly for sub Gbit/sec data rate applications and also for optical receiver circuits implemented in high-speed very large scale integrated (VLSI) technologies that typically require the use of low power-supply voltages. Thus, much greater capacitor values are needed in prior art circuits.
Certain communication protocols used or anticipated for use with optical fiber and optical receiver circuits have a worse case consecutive string of 1's or 0's which may be as long as 73.